1. Field of the Invention
This invention generally relates to a relay device which protects and controls an electric power line by detecting an electrical quantity carried by the electric power line.
2. Description of the Prior Art
In cases where the protection and controlling of the object to be controlled, such as a power line, is a function of an electrical quantity appearing on the power line, the sampling of the above-mentioned electrical quantity is achieved by using a conventional transducer.
A typical circuit diagram of a conventional transducer is shown in FIG. 1, and waveforms of portions of the circuit shown in FIG. 1 are shown in FIG. 2.
In FIG. 1, an input AC electric quantity is converted by an input transformer 11 to the level e.sub.1 which matches the circuit of the next stage.
The waveform e.sub.1 in FIG. 2 is shown for illustrative purposes as having peak values of E.sub.1l, E.sub.1m, and E.sub.1n during successive cycles. The output e.sub.1 of the input transformer 11 is full-wave rectified by a full-wave rectifying circuit 12 having four diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4. The rectified waveform is shown as e.sub.1 ' of FIG. 2.
The output e.sub.1 ' is supplied through a series resistance R.sub.L to a smoothing circuit 10 having a resistance R.sub.1 and a capacitance C.sub.1. The capacitance C.sub.1 is charged until the input AC electric quantity reaches to its peak value. After the input AC quantity has passed its peak value, the diode D.sub.1 of the full-wave rectifier 12 becomes turned OFF. Accordingly, the electric charge of the capacitance C.sub.1 is discharged by the time constant determined by the capacitance C.sub.1, the resistance R.sub.1, and the resistance component of the load to be connected to the output of the transducer.
When the input quantity e.sub.1 ' increases sufficiently in amplitude to make the diode D.sub.1 conductive, i.e. the amplitude of e.sub.1 ' exceeds the terminal voltage of the capacitance C.sub.1, the capacitance C.sub.1 is charged in accordance with the amplitude of the input quantity e.sub.1. Accordingly, the peak value of the input quantity e.sub.1 ' is maintained in the capacitance C.sub.1.
When the peak value of the input quantity changes from the value E.sub.1l to the value E.sub.1m (E.sub.1l &lt;E.sub.1m), the output of the mentioned transducer rapidly changes in response to the change in the input quantity.
However, when the peak value of the input quantity changes from the value E.sub.1m to the value E.sub.1n (E.sub.1m &gt;E.sub.1n), the response is delayed. Thus after the input quantity has reached the level of the peak value E.sub.1m, the electric charge of the capacitance C.sub.1 is discharged until the diode D.sub.1 of the full-wave rectifying circuit 12 becomes conductive.
Until the discharging curve e.sub.2, as shown in FIG. 2, reaches the next level after the peak value E.sub.1n, the output e.sub.2 of the transducer does not follow or detect the change of the input quantity e.sub.1 '. In the example of FIG. 2, the output of the transducer follows the changes in the input quantity with a delay of about two cycles.
It is understood that to reduce the ripple component of the output e.sub.2, a large time constant for discharge is set by increasing the value of the capacitance C.sub.1 or the resistance R.sub.1. However, from the above-mentioned description it is easily understood that the transducer can not rapidly follow the changes in the input quantity. Thus, it is impossible to provide high speed response in the relay device using a conventional transducer and thus it is also difficult to apply the relay device to an actual electric power line.